A touchscreen is an electronic display that detects the presence of touch and location of the touch within the field of the display. The term “touchscreen” originates from the nature of the screen that is to be touched with a finger or stylus in order to control or initiate commands displayed on the screen. Such commands can be represented on the screen in various forms such icons, words, pictures, etc. Touchscreens are typically used in devices such as tablet computers, iPads, and smart phones, e.g., iPhones, game consoles, etc.
A touchscreen enables one to interact directly with the display on the screen rather than through use of a cursor controlled by a mouse or touchpad. Furthermore, this can be done without requiring a hand-held device, except for a stylus, which sometimes may be needed for touchscreens. A touchscreen can be attached to a computer, can be used as a terminal that is connected to a network, and can be used in connection with digital devices such as mobile phones, video games, GPS devices, etc.
An essential part of a touchscreen is a touchscreen sensor which is an electronic device inherent in the visual display that is hidden from the user's view and neither obstructs nor impairs images on the screen.
There exists a great variety of touchscreen sensors for touchscreen displays that are described in many patent publications relating to construction of touch screens as well as to methods of their manufacture and use. In general, depending on the method of sensing, touchscreen sensors are divided into different groups such as resistive touchscreen sensors disclosed in U.S. Pat. No. 5,815,141 granted to Robert Phares in September 1998; U.S. Pat. No. 7,196,696 granted to Tsung-Ying Li in March 2007; U.S. Pat. No. 7,265,686 granted to G. Samuel Hurst, et al, in September 2007; U.S. Pat. No. 6,483,498 granted to Evan G. Colgan, et al, in November 2002; U.S. Pat. No. 7,196,218 granted to Ronald S. Cok, et al, in March 2007; U.S. Pat. No. 8,179,381 granted to Matthew H. Frey, et al, in May 2012; US Patent Applications 2005/0076824 published in April 2005 (inventors Eliza M. Cross, et al), and in many other patent publications.
All resistive touchscreen displays and their touch sensors may have differences in structural details, construction materials, etc., but, in general, any resistive touchscreen panel comprises several layers, the most important of which are two thin, transparent electrically resistive layers separated by a thin gap. The top screen (the screen that is touched) has a coating on the underside surface. Just beneath the coating on the underside surface is a similar resistive layer on top of its substrate. One layer has conductive connections along its sides, the other along the top and bottom. Electrical potential, which is applied to the aforementioned conductive connections, passes through one layer and is sensed at the other. When an object such as a finger or a stylus presses against the outer surface of the screen, the two layers become connected at the point of touch. As a result, at the time of contact, the panel is turned into a pair of voltage dividers. When touch occurs and the layers make contact, a resistor divider is formed across the top layer, and the voltage at the point of touch can be determined through use of a divider controller by different methods consisting of read-out signals of the divider.
A major benefit of resistive touch technology is its low cost. A disadvantage of existing resistive touchscreens is comparably poor image contrast caused by additional reflections from the extra layer of material placed over the image-producing part of the display.
The next group of touchscreen sensors for displays is a capacitive sensors. Capacitive touchscreen sensors for displays are described, e.g., in U.S. Pat. No. 6,819,316 granted to Stephen C. Shultz, et al, in November 2004; U.S. Pat. No. 6,469,267 granted to Laurence M. Welsh, et al, in October 2002; U.S. Pat. No. 6,587,097 granted to Brian E. Aufderheide, et al, in July 2003; U.S. Pat. No. 6,825,833 granted to Roger C. Mulligan, et al, in November 2004, and many others. In general, a capacitive touchscreen contains an insulator such as glass that is coated with a transparent conductor, such as indium tin oxide (ITO). Capacitive sensors of this type operate on the principle of distortion in the screen's electrostatic field when the screen is touched, e.g., by a person's finger. These changes in capacitance caused by distortions are measured in order to determine the location of touch. There are many technologies for realization of capacitive touchscreens and their sensors. For example, in devices used on the basis of surface capacitance, only one side of an insulator is coated with a conductive layer. A low voltage is applied to the layer, resulting in generation of a uniform electrostatic field. When a conductor, such as a human finger, contacts the uncoated surface, a capacitor is dynamically formed. The sensor's controller can determine the location of the touch indirectly from the changes in capacitance as measured from the four corners of the panel.
Some capacitive touchscreens are based on the principle of mutual capacitance, which makes use of the fact that most conductive objects are able to hold a charge if they are very close together. In mutual capacitive sensors, a capacitor is located at every intersection of each row and each column provided in the structure of the screen. For example, a matrix consisting of 16-by-14 arrays provides 224 independent capacitors. Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field, which reduces the mutual capacitance.
Another modification of capacitive touchscreen sensors is a self-capacitance device, which may have the same grid formed by rows and columns as the grid of mutual capacitance sensors, but the rows and columns operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. Self-capacitance produces a stronger signal than mutual capacitance, but it is not sufficiently accurate. Still another touchscreen technology, e.g., one described in U.S. Pat. No. 5,717,434 granted to Kohji Toda in February 1998, U.S. Pat. No. 5,854,450 granted to Joel Kent in August 1996, and, e.g., U.S. Pat. No. 6,091,406 granted to Shigeki Kambara, et al, in July 2000, is based on the use of surface acoustic waves, such as, e.g., ultrasonic waves, which pass over the touchscreen panel. When the panel is touched, a portion of the wave is absorbed. Such a change in the ultrasonic waves registers the position of the touch and sends this information to the controller for processing.
Among other touchscreen techniques worth mentioning is an acoustic pulse recognition system based on the principle that a touch at each position on the glass generates a unique sound, see, e.g., U.S. Pat. No. 7,315,300 granted to Nicholas P. R. Hill, et al, in January 2008, and U.S. Pat. No. 7,593,005 granted to Gokalp Bayramoglu in September 2009. Four tiny transducers attached to the edges of the touchscreen glass pick up the sound of the touch. The sound is then digitized by the controller and compared with a list of prerecorded sounds for every position on the glass. The cursor position is instantly updated to the touch location. Some systems of this type employ sensors to detect piezoelectricity in the glass, which occurs due to touch. Complex algorithms then interpret this information and provide the actual location of the touch. This technology claims to be unaffected by dust and other outside elements, including scratches. Since there is no need for additional elements on a screen, this technology also claims to provide excellent optical clarity. Also, since mechanical vibrations are used to detect a touch event, any object can be used to generate these events, including fingers and stylus. A downside is that after the initial touch, the system cannot detect a motionless finger. Also known in the art is the recently developed infrared touchscreen, see, e.g., U.S. Pat. No. 8,130,202 granted to James L. Levine, et al., in March 2012, which uses an array of X-Y infrared LED and photodetector pairs around the edges of the screen to detect a disruption in the pattern of LED beams. These LED beams cross each other in vertical and horizontal patterns. This helps the sensors to pick up the exact location of the touch. A major benefit of such a system is that it can detect essentially any input, including a bare finger, gloved finger, stylus, or pen.
Optical touchscreens are also a relatively modern development in touchscreen technology; see, e.g., US Patent Application 2011/0134036 published in June 2011 (inventor Bradley Neal Suggs). Here, two or more image sensors are placed around the edges (mostly the corners) of the screen. Infrared backlights are placed in the camera's field of view on the other side of the screen. A touch shows up as a shadow, and each pair of cameras can then be pinpointed to locate the touch or even to measure the size of the touching object. This technology is growing in popularity due to its scalability, versatility, and affordability, especially for larger units. Also known in the art are touchscreens that employ a combination of sensors based on different physical principles described above.
However, in spite of a great variety of various touchscreens and their sensors, an important problem encountered by conventional touchscreen devices is relatively high energy consumption compared with the energy consumption of old LCD and LED screens. For example, old mobile phones could last for a week or more on the same charge; typically, the standby time of such phones could exceed 360 hours over 15 days. However, the latest iPhone, which includes a touchscreen and other options, must be charged once every few days. Among other reasons, this frequent charging is associated with the fact that the energy requirement of touch sensors causes additional load on the batteries of the device, especially in hand-held devices that incorporate such sensors. For this reason, the demand exists for the development of a new generation of touch sensors that would be more efficient from the viewpoint of energy consumption.